Light emitting diode array and optical image forming apparatus with light emitting diode array

ABSTRACT

An image forming system utilizing a light emitting diode (LED array) having LEDs arranged along a curved line. The LEDs are either edge emitting type LEDs or surface emitting type LEDs. The LEDs of the LED array emit light towards a center or optical axis of the lens. A plurality of lenses are connected together to form a lens array. Alternatively, two lens arrays can be utilized. If two lens arrays are utilized, each of the lenses in the lens array includes an aspherical surface. The curved LED array prevents the flaring of light and produces an even pattern of light emission on a light receiving surface such as a photoconductive drum.

This application is a Continuation of application Ser. No. 08/744,311,filed on Nov. 7, 1996, now U.S. Pat. No. 5,838,024, issued Nov. 17,1998.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to a light emitting diode (LED) arrayfor optical printers, digital copiers, facsimile machines, etc. Moreparticularly, the present invention is directed to a light emittingdiode array in which a plurality of light emitting elements are disposedalong a curved line in order to reduce distortion.

2. Discussion of the Background

A light emitting diode array includes a plurality of light emittingelements which are disposed in at least one row. Such a light emittingdiode array is used in an optical printer, a digital copier, or afacsimile machine. The light emitting diode array is more reliable foruse under vibrations and noises than a laser beam printer. The lightemitting diode array is also suitable for miniaturizing an optical imageforming apparatus, as the light emitting diode array does not need ascanning mechanism such as a polygonal mirror as is required by thelaser beam printer.

FIG. 8 illustrates a base 80 having mounted thereon a conventional lightemitting diode array 70 with two lens arrays 90 and 100 for use in anoptical image forming apparatus. The light emitting diode array 70comprises a plurality of light emitting diodes 71 which are disposed inone row and on a straight line at the same predetermined distance fromeach other. The straight line on which the light emitting diodes 71 aredisposed is perpendicular to an optical axis of lenses 91 and 101 in thelens arrays 90 and 100.

A pattern of light emission from the light emitting diode array 70 isfocused on a photosensitive body such as a photoconductive drum throughthe two lens arrays 90 and 100 so as to form an image on thephotosensitive body. However, the above-mentioned optical image formingapparatus with light emitting diode array 70 and lens arrays 90 and 100has a drawback in that light loss occurs at connecting portions 91a,101a between each lens 91 and 101, and in that flared light occurs oneach lens 91 and 101. As a result, a pattern of light emission onto thephotosensitive body becomes uneven.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides for a novel light emittingdiode array which can solve the aforementioned drawbacks, and, thus, anobject of the present invention is to provide for a novel light emittingdiode array in which a pattern of light emission onto the photosensitivebody from the light emitting diode array and the lens array is uniform.

It is another object of the present invention to provide for a novellight emitting diode array in which the occurrence of light loss andflared light is prevented.

These and other objects are accomplished by a light emitting diode arrayfor use in an optical image forming apparatus which has the LEDsarranged along a curved line. Light from the LEDs along the curved lineis focused through a lens on a light receiving surface such as aphotoconductive drum. This forms a latent image on the photoconductivedrum which can be developed using conventional toner developingtechniques into a toner image which is transferred onto a page in aconventional manner. The LEDs are edge emitting type LEDs oralternatively surface emitting type LEDs. The LEDs at the ends of theLED array are aimed toward and project light towards the center of thelens and also towards the optical axis of the lens.

The lens used by the present invention can include one lens array or twolens arrays in parallel. If two lens arrays are used, each of the lensesinclude an aspherical surface. The arrangement of the invention can beused in an image forming system such as a copier, printer, or facsimilemachine.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the present invention and many of theattendant advantages thereof will be readily obtained as the samebecomes better understood by reference to the following detaileddescription when considered in connection with the accompanyingdrawings, wherein:

FIGS. 1A and 1B are views showing the construction of an edge emittingtype light emitting diode array in accordance with the presentinvention;

FIGS. 2A and 2B are view s showing the construction of a surfaceemitting type light emitting diode array in accordance with the presentinvention;

FIG. 3 is a view showing the first embodiment of the light emittingdiode array with a lens array for an optical image forming apparatus inaccordance with the present invention;

FIG. 4 is a view showing an enlargement of the light emitting diodearray with the lens array of FIG. 3;

FIGS. 5A, 5B, and 5C are diagrams of spherical aberration, astigmatismand distortional aberration, respectively, of the first embodiment ofthe light emitting diode array with the lens array for the optical imageforming apparatus in accordance with the present invention;

FIG. 6 is a view showing the second embodiment of the light emittingdiode array with two lens arrays for the optical image forming apparatusin accordance with the present invention;

FIGS. 7A, 7B, and 7C are diagrams of spherical aberration, astigmatismand distortional aberration, respectively, of the second embodiment ofthe light emitting diode array with two lens arrays for the opticalimage forming apparatus in accordance with the present invention; and

FIG. 8 is a view showing a conventional light emitting diode array withlens array for the optical image forming apparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of a light emitting diode array and an opticalimage forming apparatus with the light emitting diode (LED) array inaccordance with the present invention will now be explained withreference to the accompanying drawings, wherein like reference numeralsare utilized to designate identical or corresponding elements throughoutthe several views.

FIGS. 1A and 1B are views showing the construction of an edge emittingtype light emitting diode array in accordance with the presentinvention. Light emitting diodes 2 with light emitting elements 2A areprovided on a base 3. The light emitting diode array has a plurality oflight emitting diodes 2 which are disposed on a curved line with a samepredetermined distance between each LED 2. The Z direction is a lightemitting direction, and the Y direction is a direction of a rotatingaxis of the photosensitive body.

FIGS. 2A and 2B are views showing the construction of a surface emittingtype light emitting diode array in accordance with the presentinvention. A plurality of light emitting diodes 2' are provided on abase 3'. The light emitting diode array has a plurality of lightemitting diodes 2' which are disposed on a curved line at a samepredetermined distance From each other.

The edge emitting type LED array in FIG. 1 and the surface emitting typeLED array in FIG. 2 are manufactured using a semiconductor formingprocess such as the LPE (Liquid Phase Epitaxial) method or the MOCVD(Metal Organized Chemical Vapor Deposit) method. The edge emitting typelight emitting diode array is more easily manufactured than the surfaceemitting type LED array.

FIG. 3 is a view showing the first embodiment of the light emittingdiode array having a lens array for the optical image forming apparatusin accordance with the present invention, and FIG. 4 is an enlarged viewof FIG. 3. It is to be noted that the elements illustrated in FIGS. 3and 4 can correspond to similarly named elements in FIGS. 1A, 1B, 2A,and 2B. As illustrated in FIGS. 3 and 4, the light emitting diode arrays51 and 52 have a plurality of light emitting diodes 20 which aredisposed on a curved line at a same predetermined distance from eachother on a base 10. The light emitting diode arrays 51 and 52 correspondto aspherical lenses 41 and 42 of a lens array 4. The lens array 4focuses the light from the LED array 51 to a photosensitive body 50. Thedistance between the optical axes of lenses 41 and 42 are apredetermined distance P. In FIG. 3, the "R" values represent a radiusof specific curvature and the "D" values represent a distance. Numericvalues of the R and D values are provided below with respect to thefirst embodiment.

Luminous flux is emitted from the light emitting diode located at theend of the light emitting diode array 51 closest to the array 52 towardsthe optical axis of lens 41 at an angle θ (and consequently towards theoptical axis of the lens 41) onto the photosensitive body 50 so thatamounts of the luminous flux from each of the diodes 20 of the array 51to the lens 41 are equal.

As can be seen in FIG. 1A, 2A, and 3, a direction of emission of lightfrom the LEDs and a curved line connecting the LEDs are generallyplanar. For example, in FIG. 1A, the line connecting the LEDs is in theZY plane and the light from the LEDs is also in the ZY plane.Additionally, in FIG. 2A, the curve connecting the LEDs is in the ZYplane and the direction of emission of the LEDs is also in the ZY plane.

The curved line of the light emitting diode array 51 and the asphericalsurface of the lens 41 are explained herein. In the followingdescription for the aspherical surface, an x-coordinate is set to be inconformity with the optical axis of the lens, and an h-coordinate is setto be perpendicular to the optical axis of the lens.

In the equation set forth below, r designates a radius of curvature of asurface on the optical axis, k designates a conical constant, and a, b,c and d designate aspherical coefficients of higher orders. In thiscase, as is well known, the aspherical surface is provided by a curvedsurface obtained by rotating a curve represented by the followingformula around the optical axis of the lens. ##EQU1##

A shape of the aspherical surface is specified by providing the radiusof curvature on the optical axis of the lens, the conical constant andthe aspherical coefficients of higher orders.

With respect to the aspherical coefficients, E and a number subsequentto this E designate a power. For example, E-9 shows 1/10⁹ and this value1/10⁹ is multiplied by a numerical value which appears before the E.

In the following description for the curved line U on which the group oflight emitting diodes for a respective lens is disposed, a y-coordinateis set to be in conformity with the arranging direction of the lightemitting diode array. In the equation below, reference characters L, K,A, B, C and D designate coefficients. ##EQU2## The shape of the curvedline is defined by the coefficients of equation (2). As equation (2) hasa similar format to equation (1) and equation (1) defines an asphericalcurve, equation (2) also defines an aspherical curve.

In FIG. 3, the curved line of the LED array 51 includes several inherentfeatures. The curved line contains three points at which the firstderivative is zero, and two points of inflection. The definition of apoint of inflection is a point at which there is a change in curvatureof a curve from concave to convex of conversely. Further, the curvedline of the LED array 51 contains a total of 5 maxima and minima.

First Embodiment

Below are parameters of a first exemplary embodiment of the invention,as illustrated in FIG. 3.

                  TABLE I    ______________________________________    Object        Radius (mm) Index of Refraction    ______________________________________    Diode Array   1.469E-4    1.0    1st Lens Surface                  4.034       1.5721    2nd Lens Surface                  -83.729     1.0    Photosensitive                  ∞     N/A    Body    ______________________________________

D1=13.0 mm

D2=6.40 mm

D3=10.52 mm

D4=2.71

Parameters of the Aspherical Surface of the 1st Lens Surface

r=4.034, k=0.2224,

a=-1.5650E-3, b=-1.2677E-4,

c=1.3081E-5, d=-1.5695E-6

Parameters of the Aspherical Surface of the 2nd Lens Surface

r=-83.729, k=-583.6955,

a=3.5363E-3, b=3.8881E-4,

c=-2.364E-5, d=5.3886E-6

Parameters of the Curve Line

L=1.469E-4, K=-444.7470,

A=-2.2664E-2, B=5.2473E-3,

C=-1.3095E-4, D=-3.7272E-5

Other parameters of the lenses are as follows:

Focal length: 6.91 mm

Brightness of lens (F No): 3.27

Valid diameter: 5.37 mm

Distance between each lens P: 5.47 mm

Lateral magnification: -1.10

When the image resolution is 600 dots per inch, the number of lightemitting diodes in the array for each lens is lens 128 per 5.47 mm.

FIGS. 5A-5C are diagrams of spherical aberration, astigmatism anddistortional aberration of the first embodiment of the light emittingdiode array and the lens array for the optical image forming apparatusin accordance with the present invention.

In FIG. 5B, the broken (dashed) line represents the astigmatism alongthe meridional direction (the array arranging direction) and the solidline represents the astigmatism along the sagittal direction which isthe direction perpendicular to the array arranging direction. In FIG.5B, the dashed line is close to the vertical axis, thus indicating thatthis embodiment obtains a reduced meridional astigmatism.

FIG. 6 is a view showing the second embodiment of the light emittingdiode array with two lens arrays for the optical image formingapparatus. As illustrated in FIG. 6, a light emitting diode 20' isprovided on the base 10'. The light emitting diode array has a pluralityof light emitting diodes which are disposed on a curved line at a samepredetermined distance from each other. The light emitting diode arraycorresponds to two aspherical lenses 41A and 41B of a lens array 4'.Luminous flux is emitted from the light emitting diode 20' located at alowermost edge of the light emitting array toward the center of the twolenses 41A and 41B (and consequently towards their optical axes) ontothe photosensitive body 50.

The curved line for the light emitting diode array and the asphericalsurface of two lenses 41A and 41B are explained hereinafter.

Second Embodiment

Below are parameters of a second exemplary embodiment of the inventionas illustrated in FIG. 6.

                  TABLE II    ______________________________________                                 Index of    Object          Radius (mm)  Refraction    ______________________________________    Diode Array     -1.958E-8    1.0    1st Lens Surface                    -29.847      1.5721    2nd Lens Surface                    ∞      1.0    3rd Lens Surface                    ∞      1.5721    4th Lens Surface                    -4.661       1.0    Photosensitive  ∞      N/A    Body    ______________________________________

D1=11.00 mm

D2=2.50 mm

D3=2.00 mm

D4=2.5 mm

D5=20.00 mm

D6=2.71 mm

Spherical Surface of 1st Lens Surface

r=-29.847, k=-504.7168,

a=-3.4307E-3, b=-1.8331E-4,

c=-5.6715E-5, d=1.1841E=5

Aspherical Surface of 4th Lens Surface

r=-4.661, k=-1.2592,

a=6.3732E-4, b=-1.6292E-4,

c=-1.5340E-5, d=2.3309E-6

Curved Line

L=-1.958E-8, K=-7.1770E+12

A=2.6166E-2, B=-1.2259E-2,

C=1.2925E-3, D=2.3840E-5

Focal length: 8.64 mm

Brightness of lens (F No): 3.67

Valid diameter: 5.40 mm

Distance between each lens P: 5.69 mm

Lateral magnification: -1.10

When the image resolution is 600 dots per inch, the number of lightemitting diodes in the array is 134 per 5.69 mm.

FIGS. 7A-7C are diagrams of spherical aberration, astigmatism, anddistortional aberration, respectively, of the second embodiment of thelight emitting diode array with two lens arrays for the optical imageforming apparatus in accordance with the present invention. In theseFigures, the astigmatism in the meridional direction, corresponding tothe broken line in FIG. 7B and the distortional aberration illustratedin FIG. 7C are improved by arranging the LEDs on the curved line.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein.

What is claimed as new and desired to be secured by Letters Patent of the United States is:
 1. A light emitting diode, "LED", system, comprising:an LED array having LEDs disposed thereon along a curved line, each of the LEDs forming the curved line, and a direction of emission of the LEDs and the curved line being generally planar; and a lens which focuses light emitted from the LEDs.
 2. A system according to claim 1, wherein:the LED array includes edge emitting type LEDs.
 3. A system according to claim 1, wherein:the LED array includes surface emitting type LEDs.
 4. A system according to claim 1, wherein:the LEDs of the LED array emit light towards an optical axis of the lens.
 5. A system according to claim 1, wherein:the lens includes two aspherical surfaces.
 6. A system according to claim 1, further comprising:a second lens which receives light from the first lens and focuses light from the first lens onto the light receiving surface.
 7. A system according to claim 6, wherein:each of the lenses includes an aspherical surface.
 8. A system according to claim 1, wherein:the curved line is an aspherical line.
 9. A system according to claim 1, wherein:the lens has at least one curved surface which is an aspherical surface.
 10. A system according to claim 9, wherein:the curved line which is an aspherical line is arranged to reduce an astigmatism of the system and the curved surface which is an aspherical surface is arranged to reduce aberrations.
 11. A system according to claim 1, wherein:the curved line has LEDs at ends thereof aimed at angles towards a center of the lens.
 12. A system according to claim 1, wherein:an LED at a center of the LED array is aimed towards a center of the lens.
 13. A system according to claim 12, wherein:the curved line has LEDs at ends thereof aimed at angles towards a center of the lens.
 14. A system according to claim 1, wherein:the array is parallel to an axis of rotation of a light receiving surface.
 15. A system according to claim 1, wherein:the curved line has two inflection points.
 16. A system according to claim 15, further comprising:a first axis parallel to a rotating axis of the light receiving body; and a second axis perpendicular to the first axis, wherein the curved line has at least a total of five local minima and maxima relative to the second axis.
 17. A system according to claim 1, wherein:the curved line comprises three points at which a first derivative of the line is zero.
 18. A system, comprising:an LED, Light Emitting Diode, array means having LEDs disposed thereon along a curved line, each of the LEDs forming the curved line, the LED array means for emitting light, and a direction of emission of the LEDs and the curved line being generally planar; and a lens means for focusing light emitted from the LEDs.
 19. A system according to claim 18, wherein:the LED array means includes edge emitting type LED means.
 20. A system according to claim 18, wherein:the LED array means includes surface emitting type LED means.
 21. A system according to claim 18, wherein:the LEDs of the LED array means emit light towards an optical axis of the lens means.
 22. A system according to claim 18, wherein:the lens means includes two aspherical surfaces.
 23. A system according to claim 18, further comprising:a second lens means which receives light from the first lens means and focuses light from the first lens means onto a light receiving means.
 24. A system according to claim 23, wherein:each of the lense means includes an aspherical surface.
 25. A system according to claim 18, wherein:the curved line is an aspherical line.
 26. A system according to claim 25, where:the lens means has at least one curved surface which is an aspherical surface.
 27. A system according to claim 26, wherein:the curved line which is an aspherical line is arranged to reduce an astigmatism of the system and the curved surface which is an aspherical surface is arranged to reduce aberrations.
 28. A system according to claim 18, wherein:the curved line has LEDs at ends thereof aimed at angles towards a center of the lens means.
 29. A system according to claim 18, wherein:an LED means at a center of the LED array means is aimed towards a center of the lens means.
 30. A system according to claim 29, wherein:the curved line has LED means at ends thereof aimed at angles towards a center of the lens means.
 31. A system according to claim 18, wherein:the array means is parallel to an axis of rotation of a photosensitive means.
 32. A system according to claim 18, wherein:the curved line has two inflection points.
 33. A system according to claim 32, further comprising:a first axis parallel to a rotating axis of a photosensitive means; and a second axis perpendicular to the first axis, wherein the curved line has at least a total of five local minima and maxima relative to the second axis.
 34. A system according to claim 18, wherein:the curved line comprises three points at which a first derivative of the line is zero. 